Shafts of golf clubs conventionally include those made of steel and those made of a fiber-reinforced resin (hereinafter, referred to as FRP). In general, a golf club equipped with a steel shaft provides an advantage that directional accuracy is stabilized, and a golf club equipped with an FRP shaft provides an advantage that a weight reduction can be achieved, thus increasing a swing speed and improving a launch angle and a carry.
Such an FRP shaft is formed by winding a plurality of prepreg sheets on a mandrel, the plurality of prepreg sheets including reinforcing fibers impregnated with a synthetic resin, and thermally curing them, followed by de-coring. While there are a wide variety of ways of configuring the prepreg sheets wound on the mandrel, for example, as disclosed in Japanese Patent Application Publication No. 2012-245309 (the '309 Publication), in a portion to which a head is mounted, a prepreg sheet for reinforcement (a prepreg sheet including reinforcing fibers oriented in an axial direction) is wound over a given distance (about 300 mm from a distal end thereof). The purpose of this is as follows. The shaft is formed so as to be decreased in diameter near a distal end thereof, and therefore, in the portion to which the head is mounted, an outer surface of the shaft and an inner surface of a fitting hole of a hosel portion of the head are fixed to each other so as to form a straight shape, and this fixing region and a vicinity thereof are prevented from being decreased in strength.
Conceivably, the reason why a golf club equipped with a steel shaft exhibits excellent directional accuracy in ball hitting as described above is that metal has an isotropic property, so that a ratio between torsional rigidity and bending rigidity (GIp/EI) of the steel shaft is uniform over a length direction thereof. In contrast, as for an FRP shaft, while it provides an advantage that, compared with a steel shaft, a weight reduction can be achieved, thus increasing a swing speed and improving a launch angle and a carry, FRP has longitudinal and transverse elastic moduli varying depending on an orientation of reinforcing fibers and thus has an anisotropic property, so that a ratio between torsional rigidity and bending rigidity (GIp/EI) of the FRP shaft varies over a length direction thereof. Conceivably, this is the reason for not being able to stabilize directional accuracy as much as a steel shaft does.
In the FRP shaft disclosed in the '309 Publication, as described above, the prepreg sheet for reinforcement (hereinafter, referred to as a reinforcement sheet) including reinforcing fibers oriented in the axial direction is arranged in a distal end region to which the head is mounted, ending up causing a problem in stabilizing directional accuracy. That is, in a case where such a reinforcement sheet is arranged, bending rigidity is increased in the distal end region, and since the number of winding turns increases toward the distal end, as shown in FIG. 1A where a horizontal axis (mm) represents a longitudinal direction of the shaft and a vertical axis (Kgf·mm2) represents bending rigidity, there is obtained a bending rigidity distribution in which the bending rigidity has an inflection point in a neighborhood of 300 mm from the distal end and increases toward the distal end. In this case, the reason why the bending rigidity is lowest in the neighborhood of 300 mm from the distal end is that when the reinforcement sheet has a length of about 300 mm, this part forms an end portion of the reinforcement sheet (the number of winding turns is 0). In a case where the above-described reinforcement sheet is not arranged, the bending rigidity directly decreases toward the distal end with no inflection point occurring in the neighborhood of 300 mm.
The reinforcement sheet thus wound includes reinforcing fibers oriented in the axial direction and thus significantly affects bending rigidity, while directionality thereof hardly affects torsional rigidity. Because of this, similarly to FIG. 1A, there is obtained a torsional rigidity distribution as shown in FIG. 1B where a horizontal axis (mm) represents the longitudinal direction of the shaft and a vertical axis (Kgf·mm2) represents torsional rigidity.
Accordingly, in the FRP shaft with the reinforcement sheet wound thereon, the reinforcement sheet including reinforcing fibers oriented in the axial direction, a variation of the ratio between torsional rigidity and bending rigidity (GIp/EI) is increased in the distal end region (the ratio between torsional rigidity and bending rigidity varies over the length direction thereof), leading to a problem in stabilizing directional accuracy. Particularly in playing golf by use of a golf club set composed of a golf club equipped with a steel shaft and a golf club equipped with an FRP shaft, there occurs a difference in feeling of bowing and feeling of torsion between these both types of golf clubs, making a miss-shot likely to occur. Further, while achieving a weight reduction and thus providing ease of swinging, the golf club equipped with an FRP shaft makes it difficult to stabilize directional accuracy.